MMC: S3C24XX: Add media presence test to request handling.

Ensure that we have physical media present before attempting to
send a request to a card. This ensures that we do not get flooded
by errors from commands that can never be completed timing out.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>